Many doors, such as household doors, are provided with a dead bolt lock in order to maintain security by bolting the door shut. Incidentally, dead bolts are normally required to extend one inch into the door frame and associated latch plate. Unfortunately, the dead bolt must be released and the door opened in order to (1) pass a small object through the doorway, such as an envelope or small package, or (2) to view the persons outside the door. While a short chain may be used to prevent the door from fully opening, such a chain arrangement is often weak.
An alternative to a standard dead bolt lock is a lock that can operate either in a mode similar to a dead bolt or as a latch having a "Z-Bar" configuration, which allows the door to open a limited distance. Such a lock is disclosed in U.S. Pat. No. 3,924,887. That device employs a baseplate which attaches to the door; an intermediate link; an outer link with a latch disk and a mode selection handle; and a latch plate mounted on the door frame into which the latch disk can latch.
However, there are at least four problems with the device. First, the device employs rounded "ears" to hold the latching assembly together in the dead bolt mode. As structural members, these "ears" are weak and can compromise the strength of the lock. Second, when the lock is in the dead bolt mode or limited-open mode, the latch disk can be pried out directly away from the latch, for instance by someone using a crowbar. Third, since the latch disk is the primary load-bearing portion of the lock, the latch disk should be as thick as possible. However, when the entire latch disk of U.S. Pat. No. 3,924,887 is made thick, the entire latch mechanism must increase in size to accommodate it.
Finally, the prior device does not provide for an indicator to inform the user what mode the lock is in. Without such an indicator, the user may unwittingly believe that the door has been dead bolted shut, when in fact it is unlocked or in the limited-open position.
It is also noted in passing that the related U.S. Pat. No. 3,924,886 discloses an alternative embodiment in FIGS. 7-10 of the patent, which only provides a sketchy and brief description, and does not have an intended dead bolt mode of operation.